Synthetic Aperture Radar, SAR, as such is a known technique, by which it is possible with a radar device mounted on a moving platform, normally an aircraft or a satellite, to obtain high resolution images of the ground. Radar responses from the ground are stored during some interval of the flight of the platform. The SAR image is obtained by signal processing in ways similar to computer tomography. Image resolution is determined by the angular span of viewing angles of the imaged ground, as well as the wavelength used and the distance between the radar and the ground. This means that the actual resolution of the radar antenna is of no importance for the resolution of the obtained image.
A SAR is preferably used from air though ground based systems are also feasible. An airborne SAR produces two-dimensional images perpendicular to the aircraft path of flight. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: Range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted signal bandwidth, i.e. large bandwidth signals yield fine range resolution.
The other dimension is called azimuth (or along track) and is perpendicular to range over the ground surface. It is the ability of SAR to produce fine azimuth resolution that differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in frequency than optical systems, even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar could collect data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna.
While this section attempts to provide an intuitive understanding, SARs are not as simple as described above. For even moderate azimuth resolutions, a target's range to each location on the synthetic aperture changes along the synthetic aperture. In SAR the energy reflected from the target is “mathematically focused” to compensate for the range dependence across the aperture prior to image formation. When the aperture is large the SAR can give resolution near the radar wavelength. The focusing is highly sensitive to geometry assumptions and objects will vanish in the SAR image unless these assumptions are made correctly.
Very High Frequency, VHF, SAR enables efficient detection of targets of vehicle size or larger in situations where the targets are concealed by a foliage layer or artificial camouflage. In fact, foliage attenuation is negligible below 100 MHz (it is also tolerably low at frequencies in the Ultra High Frequency, UHF band, but becomes prohibitive at the L band and higher). Another advantage of frequencies below 100 MHz is that the terrain clutter levels are significantly smaller than at all higher frequencies, including UHF. Finally, radar cross section for vehicle targets is relatively isotropic below 100 MHz (targets size being close to resonance length) whereas for UHF in particular, target response tends to be highly directive. There is a risk for a very small Radar Cross-Section, RCS, if the target is oriented in an oblique position with respect to the radar. The advantages with the VHF band have been experimentally corroborated by the CARABAS VHF/UHF SAR development, which has been carried out over a time span of some 20 years.
A disadvantage with VHF is that resolution never can be finer than moderate; around 2 m. This number is in fact the resolution diffraction limit for SAR using ultra wide band methods but confined to frequencies below 100 MHz. Rather than to rely on resolution, target detection is better achieved at VHF (and also at UHF) by change methods comparing a prior SAR image (without targets) to a posterior image (with targets). As it turns out, VHF SAR images are temporally highly stable. Therefore, and since resolution is moderate, it is feasible to keep reference data records for large regions to be combined with new data at the time and location of a development, which calls for surveillance and target detection.
Any change detection method typically contains three steps, namely:                1. alignment of reference and update (also known as mission) images;        2. formation of a weighted difference image;        3. target detection from requirements on probability of detection and false alarm rate.        
The method can be coherent—the difference image is between weighted complex image amplitudes—or incoherent—the difference is formed between the modulus of the amplitudes. From investigations done, there is little evident advantage of using coherent methods. This conclusion is for foliage penetration—Saab has developed ground penetration methods where coherency is crucial.
Geo-referencing to sub resolution accuracy is a standard routine for the VHF SAR imagery produced with the current CARABAS III VHF/UHF SAR. The accuracy is sufficient for non-coherent change detection.
A conventional automatic target detection method in a SAR image is constant false alarm rate, CFAR, which is based only on the amplitude. CFAR determines the power threshold above which any return can be considered to probably originate from a target. In other words, targets are detected by determining a threshold value of the amplitudes in the image and saying that all detected amplitudes above that value shows a target. By using this method, it is very likely that some determined targets are in fact only clutter that shows a high amplitude.
The sensitivity of the radar is balanced considering both the number of targets detected and how many false alarms arises from high amplitudes due to clutter. It is not common that the current methods only find 7 out of 10 true targets. Almost each CFAR brings a large number of false alarms. Currently, just given the SAR image, there is no way of knowing the probability that a detected target is a real target or a false alarm.
There is a need to improve the target detection in the images so that the number of detected targets is closer to the true number of targets actually present in the image scene and so that the number of false alarms is reduced. Furthermore, target detection in images would be greatly improved if, within a given image, there was a way to know the probability that a detected target was a true target.